Pope Francis has called for the worldwide abolition of the death penalty, saying the commandment "You shall not kill" was absolute and equally valid for the guilty as for the innocent.

Using some of his strongest words ever against capital punishment, he also called on Catholic politicians worldwide to make "a courageous and exemplary gesture" by seeking a moratorium on executions during the Church's current Holy Year, which ends in November.

"I appeal to the consciences of those who govern to reach an international consensus to abolish the death penalty," he told tens of thousands of people in St. Peter's Square.

"The commandment "You shall not kill," has absolute value and applies to both the innocent and the guilty," he told the crowd.

The 1.2 billion-member Catholic Church allowed the death penalty in extreme cases for centuries, but the position began to change under the late Pope John Paul, who died in 2005.

The pope added that there was now "a growing opposition to the death penalty even for the legitimate defence of society" because modern means existed to "efficiently repress crime without definitively denying the person who committed it the possibility of rehabilitating themselves."

Francis made the comments to throw his weight behind an international conference against the death penalty starting Monday in Rome and organised by the Sant'Egidio Community, a worldwide Catholic peace and justice group.

Francis, who has visited a number of jails since his election as pope nearly three years ago - the latest in Mexico last week - also called for better prison conditions.

"All Christians and men of good will are called on to work not only for the abolition of the death penalty, but also to improve prison conditions so that they respect the human dignity of people who have been deprived of their freedom," he said.

In the past, the pope also denounced life imprisonment, calling it "a hidden death penalty" and saying that more should be done to try to rehabilitate even the most hardened of criminals.